Systematic design and realization of double-negative acoustic metamaterials by topology optimization

Highlights: • Unified topology optimization framework is developed for designing double-negative acoustic metamaterials (AMMs). • Representative resonance-cavity-based and space-coiling microstructures are explored. • Broadband double negativity originating from novel multipolar LC or Mie resonances can be induced systematically. • Desired broadband subwavelength imaging of topology-optimized AMMs is verified experimentally. -- Abstract: Double-negative acoustic metamaterials (AMMs) offer the promising ability of superlensing for applications in ultrasonography, biomedical sensing and nondestructive evaluation. However, the systematic design and realization of broadband double-negative AMMs are stilling missing, which hinder their practical implementations. In this paper, under the simultaneous increasing or non-increasing mechanisms, we develop a unified topology optimization framework involving different microstructure symmetries, minimal structural feature sizes and dispersion extents of effective parameters. The optimization framework is applied to conceive the heuristic resonance-cavity-based and space-coiling metamaterials with broadband double negativity. Meanwhile, we demonstrate the essences of double negativity derived from the novel artificial multipolar LC (inductor-capacitor circuit) and Mie resonances which can be induced by controlling mechanisms in optimization. Furthermore, abundant numerical simulations validate the corresponding double negativity, negative refraction, enhancement of evanescent waves and subwavelengh imaging. Finally, we experimentally show the desired broadband subwavelengh imaging by using the 3D-printed optimized space-coiling metamaterial. The present design methodology provides an ideal approach for constructing the constituent "atoms" of metamaterials according to any artificial physical and structural requirements. In addition, the optimized broadband AMMs and superlens lay the structural foundations of subwavelengh imaging technology.